Methods

Subscribes to the Overlay based instance's blurEvent to allow the OverlayManager to
monitor blur state.
If the instance already has a blurEvent (e.g. Menu), OverlayManager will subscribe
to the existing blurEvent, however if a blurEvent or blur method does not exist
on the instance, the _bindBlur method will add them, and the blur method
update the OverlayManager's state directly.

Subscribes to the Overlay based instance focusEvent, to allow the OverlayManager to
monitor focus state.
If the instance already has a focusEvent (e.g. Menu), OverlayManager will subscribe
to the existing focusEvent, however if a focusEvent or focus method does not exist
on the instance, the _bindFocus method will add them, and the focus method will
update the OverlayManager's state directly.